The present invention is directed to a hollow crown drill with the leading or cutting end face in the drilling direction equipped with at least one set of cutting members having at least two cutting members. The cutting members of each set have cutting faces of different sizes oriented in the rotational direction of the drill and the cutting members within the set are arranged so that the cutting members increase in size counter to the rotational direction.
Hollow crown drills of the most varied constructions are used for drilling boreholes of larger diameter in structural components, such crown drills subject to purely rotational drive are specially utilized in cutting larger bores in masonry structural components. In drilling such boreholes it is necessary to provide hollow crown drills capable of operating without the use of a liquid cooling medium.
A hollow crown drill is disclosed in DE-OS 36 19 334 with a leading or cutting end face having cutting members with exposed cutting faces. In this known hollow crown drill, the cutting members are made from wear-resistant material. In addition, the cutting faces effectively carrying out the cutting operation increase in size counter to the rotational direction, whereby each cutting member being used performs its share of the chip removing operation.
While in the above-mentioned hollow crown drill wear-resistant hard material is proposed as the material for the cutting members, it was noted when drilling masonry that the cutting members using polycrystalline diamond platelets are suitable. Such diamond platelets are bonded in the hard metal basic member in polycrystalline form or as coated diamonds.
The cutting members of the above-mentioned type, however, require a sufficient penetration into the material being drilled, for instance masonry, so that no excessive wear occurs. Depending on the contact pressure applied, satisfactory penetration of the hollow crown drill must be assured, so that each cutting member participates in the drilling operation by adequate penetration into the material being drilled.
The hollow crown drill in DE-OS 36 19 334 has attachments in the above-mentioned sense. In such a drill, however, wedge-shaped cutters are used, disposed in different orientations, whereby cutting members acting as roughing cutting members remove only a portion of the material and the following cutting members effect further drilling. In this known solution, the problem is solved by identically designed though differently oriented cutters with the result that certain excess stresses develop, since in one portion of the cutting members only the tips and in another portion of the cutting members only the corners are being utilized. As a result, early wear and failure of the hollow crown drill has been experienced.